The present invention relates, in general, to broadcasting information from a plurality of sources to one or more receivers. More particularly, the present invention is directed to broadcasting a digital information waveform in the same band and on the same channel with a conventional analog waveform.
At present, the sound quality of audio programming over commercial analog frequency modulation (FM) broadcast facilities is significantly poorer than that of more modern digital signal sources such as the compact disc. A number of attempts have been made to bring the quality of digital audio to FM broadcasting, but these attempts have usually given rise to other problems which rendered them unworkable.
For example, U.S. Pat. No. 5,038,402 to Robbins discloses an apparatus and method for broadcasting digital audio over the FM broadcast band and suggests that such digital broadcasts might be interspersed with analog broadcasts, across the band. This patent allows the use of the FM band for digital broadcast, but forces the individual broadcaster to choose between broadcasting in digital, with better audio but a listener base of only the new relatively scarce digital receivers, or conventional analog, with poorer audio quality but available to all listeners with conventional analog FM receivers. The only other alternative is for the broadcaster to broadcast on two frequencies, one for digital and a second for analog; however, this presents a potential problem in obtaining a license for such broadcasting from the Federal Communications Commission (FCC). The broadcaster may or may not be able to acquire a license to broadcast on two frequencies in a given FM radio broadcasting market.
The FCC, in addition to licensing individual frequency bands to individual broadcasters, is charged with allocation of frequency spectrum to all users for all uses. One other problem presented by the system of the Robbins patent and others like it is that spectrum must be allocated especially for digital FM broadcast if the number of analog broadcasters presently on the air is to remain unchanged. This problem is exacerbated by the fact that, for most of the United States and at attainable frequencies, there is no unused spectrum. In order for spectrum to be re-allocated for a new use, the FCC and the petitioners desiring use of a frequency band must go through a protracted, uncertain, political process which will culminate in an FCC decision on how the spectrum in question should be used.
For these reasons, there has been a need for digital audio broadcasting over the FM band which does not require a broadcaster to abandon its investment in analog FM transmission equipment, require a new frequency assignment in the existing FM band, require the listening audience to discard existing analog FM receivers, or force the FM program broadcasters to undergo a protracted, uncertain and expensive process to obtain a new frequency allocation.
It is, therefore, an object of the present invention to provide a system for digital audio broadcasting on the existing FM broadcast band. More particularly, the purpose of this invention is to provide a system for In-band, On-channel, FM Digital Audio Broadcast (IBOC FM-DAB) which would allow simultaneous transmission of DAB and FM over existing allocations without interfering with the conventional analog FM signals. Such a system not only would be of great value to the broadcast industry, but the ability to multiplex supplemental message information over conventional analog FM transmissions would be of general interest and importance from a perspective of efficient spectrum utilization. In addition, a system which solves the problems raised for FM DAB will inherently find applications in many other communications"" environments.
Once the requirement for an in-band, on-channel digital audio broadcast is defined, a number of subsidiary problems arise in executing that broadcast. In particular, the digital signal and the FM program must be modulated together in such a way that they can be demodulated and used by various end users on the receiver end. This causes problems of receiver design, which are made more complex since allowance must be made for disturbances which occur in the communications channel between the transmitter and receiver. For example, in FM transmission, demodulation is often inhibited by multipath, which applies fast, undesired phase changes to FM signals thereby causing a loss of phase lock in conventional receivers which normally use xe2x80x9cphase locked loopsxe2x80x9d (PLLs). Reacquisition of lock with a conventional PLL causes an undesired spurious time response at the demodulation output of the receiver, which adversely affects audio quality.
In one aspect, this invention is directed to a method for introducing supplemental programming (more messages) to a given FM broadcast by amplitude modulating the FM waveform. The supplemental amplitude modulation is orthogonal to the initial frequency modulation so that both the AM and FM programs can be demodulated, either independently or together, without interfering with one another. This method thus provides a vehicle for the simultaneous transmission of supplemental programming, such as high bit rate DAB, with existing FM over the same spectral allocation at the same time without degrading the analog FM transmission. This invention is applicable to any system in which additional programming or an additional message is to be added to enhance or supplement another program which frequency modulates a carrier.
More specifically, the present invention is directed to a method and apparatus for modulating a DAB signal, consisting of 21 digital carriers, onto an analog FM carrier such that mirror images of the 21 digital carriers are spaced in frequency on either side of the analog FM carrier. These DAB carriers slew through the frequency band at the instantaneous rate of the analog FM carrier. This slewing in frequency is accomplished without causing interference between the analog FM audio and digital subcarrier signals. The analog FM program signal then becomes, in a sense, a carrier for the 21 digital subchannels. By slewing through frequency, the composite signal becomes resistant to multipath distortion. Further resistance to multipath is derived through the addition of a continuously transmitted wideband reference signal to the 21 digital subchannel modulation waveforms. This reference waveform is used at the receiver as a training system for adaptive multipath equalization with quick and continuous updating. Additional resistance to multipath distortion comes from the use of data interleaving and data coding systems which are customarily used to detect and correct errors in digital signal systems. As a result, the FM DAB transmission system of the invention provides unusually good resistance to multipath induced distortion. The FM DAB signal is modulated at a significantly lower peak power and is positioned within the allocated spectrum mask which is licensed to each broadcaster. This allows each broadcaster to transmit the digital signal within the licensed frequency on a single FM channel.
An important consideration in the development of an in-band, on-channel FM digital audio broadcasting (FM-DAB) system is the requirement that the digital signals not interfere with the analog FM signals occupying the same frequency allocation. An in-band, on-channel FM-DAB signal simultaneously occupies the same frequency allocation as a conventional analog FM broadcast signal. The characteristics of the digital signal must therefore be designed to prevent degradation of the analog signal. One approach to minimizing interference is to reduce the amplitude of the digital signal relative to the analog signal. Of course, the amplitude of the digital signal cannot be made arbitrarily small, because interference from the analog signal and thermal noise will ultimately degrade the digital signal. Once the digital signal amplitude has been reduced to the smallest possible level which maintains the desired bit error rate over the desired coverage area, then another technique must be used to ensure non-interference with the analog signal. One such technique is to design the frequency-domain characteristics of the digital signal such that it is orthogonal to the analog signal.
Digital signals occupying the same frequency allocation as analog FM signals add a random-noise component to the received signal. A 50-dB audio (post-detection) signal-to-noise ratio (SNR) requires between 30 and 50 dB of xe2x80x9cprotectionxe2x80x9d against digital quadrature phase-shift keyed (QPSK) and quadrature amplitude modulated (QAM) signals. The wide variation in these protection ratios arises from differing test methods; a CCIR-recommended quasi-peak detection approach gives protection ratios of 38.5 to 48.5 dB to maintain 50-dB audio SNR in the presence of co-channel digital 256-QAM and QPSK signals. An alternative approach, using an RMS detection to measure xe2x80x9cunweightedxe2x80x9d SNR, yields protection ratios of 30 to 32 dB for co-channel 256-QAM digital signals, although significant audio degradation may occur at these levels.
Additional suppression of interference may be achieved by modulating the digital signal in a way that ensures that it is orthogonal to the analog signal. One method of achieving this orthogonality is to design the digital signal spectrum such that it is never superimposed directly on the analog signal spectrum. While frequency separation may not seem feasible for in-band, on-channel systems, it may be accomplished in systems which employ frequency sliding of the digital signal. In this approach, the center frequencies of the digital carriers are modulated by the FM program. This allows the correlation between the digital and the analog signals to be minimized, which also minimizes the mutual interference between the signals.
Of course, practical system implementations cannot be expected to maintain perfect orthogonality, and any correction between the analog and digital signals will result in some amount of mutual interference between the signals. The amount of interference will depend on the ability to prevent any overlap between the analog signal spectrum and the digital signal spectrum. By proper design and implementation of the digital waveform, clear-channel interference suppression of 10 to 20 dB is readily accomplished. Thus the protection ratio of 40 to 50 dB may be achieved by transmitting the digital signals approximately 30 dB below the analog carrier level.
Minimization of the correlation between the analog and digital signals in a clear (multipath-free) channel does not guarantee minimal interference in the presence of multipath. If the direct-path signal is made orthogonal to the analog signal, the delayed-path signal will not be perfectly orthogonal, and the amount of interference between the signals will depend on the frequency difference (which is proportional to the delay time of the echo and FM rate). To minimize interference in this case, the separation between the analog signal spectrum and the digital signal spectrum must be made sufficiently large to maintain orthogonality even in high-multipath environments. For multipath delay spreads of 1 to 5 microseconds, this separation must be between 5 and 30 kHz for 100 percent modulation. For extreme multipath environments, with sufficient delay spread to cause loss of orthogonality between the analog and digital signals, the analog FM signal will be degraded by the multipath to the point at which the decrease in signal-to-noise ratio caused by the digital signal is expected to be imperceptible at the audio output of the receiver.
Another aspect of the invention includes provision of receivers which will demodulate either the analog FM program, the digital DAB program, or both. In one embodiment of the receiver, the transmitted digital audio signal is extracted from the standard analog FM signal on which it is carried by a programmable notch filter based upon acoustic charged transport (ACT) technology. The ACT-based receiver optimizes the passage of the desired (digital) signal while suppressing the undesired (analog) signal.
In addition to CD quality stereo programming and improved data services, Digital Audio Broadcast (DAB) promises to mitigate the adverse impact of multipath. In-band, on-channel FM DAB delivers CD-quality audio within existing spectral allocations, while not interfering with existing FM broadcast reception. Most measures proposed for mitigating multipath involve the use of new spectrum.
Multipath is the time domain phenomenon wherein successively delayed versions of a broadcast signal arrive at the receiver simultaneously. Multipath is typically random and time variant. A multipath channel time response has an associated frequency response. Multipath is usually characterized in the frequency domain in terms of amplitude fade depth, spatial and temporal correlation of fade depths, and frequency coherency which relates to fade bandwidth. Techniques employed for mitigating multipath include: spread spectrum modulation; data encoding; frequency division multiplexing; adaptive channel equalization; and, time, frequency and spatial diversity.
Spatial diversity in the form of multiple antennas has been shown to be helpful in improving FM reception in automobiles; however, due to practical and aesthetic considerations, the use of multiple antennas has not been accepted by the FM radio industry and is not considered a part of the solution for multipath mitigation. Spread spectrum has been clearly shown to alleviate multipath, but bandwidth requirements for IBOC DAB are not consistent with existing spectral allocations for FM.
Frequency diversity becomes effective against multipath as spectral separation employed begins to exceed multipath coherence bandwidths. Urban FM multipath is thought to have coherence bandwidths in the 30 to 300 kHz range, and is thought to be resistant to in-band on-channel frequency diversity techniques; however, FCC 73.317 defines the spectral allocation for commercial FM in the United States over a 1.2 MHz bandwidth. Compliance with FCC 73.317 allows the power within 480 kHz of this bandwidth to reach 25 dBc. Using some of this power for IBOC DAB allows for a level of frequency diversity which is exploited towards the mitigation of multipath.
Adaptive channel equalization has been shown to improve multipath reception in radio systems, but the rapidly varying nature of multipath in automobiles precludes the use of conventional adaptive equalization techniques. Frequency division multiplexing, data encoding and time diversity complement frequency diversity measures and ACT-based equalization techniques provide a comprehensive in-band on-channel FM DAB system with surprisingly high multipath resistance.
Three measures inherent to the modulation method are employed to mitigate multipath: a frequency slide technique, frequency division multiplexing, and an ACT-based equalization technique.
xe2x80x9cFrequency slidingxe2x80x9d is a modulation technique in which the carrier frequencies of a series of digital subchannels are modulated by the FM program. This has the effect of producing a constant frequency offset between the analog-FM carrier and the IBOC digital signals. The primary motivation for IBOC DAB frequency slide is that sliding the DAB carrier frequencies in synchronization with the instantaneous FM signal frequency may be used to make conventional FM detection techniques insensitive to IBOC DAB. The added benefit of frequency slide is multipath mitigation. Frequency slide increases the effective IBOC DAB bandwidth for multipath mitigation without increasing the IBOC DAB noise bandwidth. Frequency slide contributes a level of effective frequency diversity against multipath.
Frequency division multiplexing is a common practice for mitigating multipath. The time domain advantage of frequency division multiplexing is the reduction of intersymbol interference (ISI) in each subchannel due to multipath with respect to the ISI which would otherwise be seen by the proportionally shorter duration symbols on a single carrier. The frequency domain advantage of frequency division multipath is the isolation of the effects of narrowband fading to a fraction of the subchannels. The errors induced on the affected subchannels are recovered through data decoding.
The ACT-based equalization technique is used to compensate for nonuniform phase distortion induced by multipath across the band. This measure allows for the coherent contribution of delayed signal components to the digital demodulation process. All the delayed signal components contribute coherently to the demodulation of each data symbol. The processing gains are analogous to those of an ideal channel equalizer with no adaptation time.
Data encoding and error correction are the subject of substantial research efforts for a variety of communications and data storage applications, and the power and efficiency of these techniques have increased significantly as a result of these efforts. Three measures inherent to the data encoding technique may be used to mitigate the effects of multipath: block coding, convolutional coding, and data interleaving.
Block coding is used to detect and correct errors. A portion of the errors due to narrowband fades or to temporary fades in a moving vehicle may be detected and corrected through block coding. Block coding is also referred to as xe2x80x9cerror detection and correctionxe2x80x9d.
Convolutional coding provides processing gain against losses of signal level through soft decision Viterbi decoding. Convolutional coding also distributes information across subchannels which adds a level of effective frequency diversity to the modulation. Soft decision Viterbi decoding essentially gives the decoder the demodulation information as well as subchannel reliability information. Data is decoded according to a set of relative subchannel confidence metrics.
Data interleaving is used to distribute burst errors between levels of coding so as to make burst errors appear random. Although convolutional encoding adds processing gain to the demodulation process, errors which do propagate through soft decision Viterbi decoding are usually bursty in nature. Interleaving spreads out burst errors in time so as to enable correction by the block decoder.
The DAB receiver includes a frequency tracking delay element interference canceler and FM demodulator which removes a dominant tone or FM interference signal by subtracting it from a delayed replica. Cancellation is maintained through a time delay that tracks the instantaneous frequency of the dominant FM interfering waveform. A delay is generated, accurately controlled and dynamically adjusted in response to changes in the instantaneous frequency of the dominant tone or FM interference signal. A phase or phase threshold detector is used to track small errors in cancellation phase in order to close the loop on the tracking canceler and to make the tracking canceler resistant to multipath. The control voltage at the adjustable delay element varies with the instantaneous frequency of the predominant tone. In the case where an FM signal is tracked and cancelled, the control voltage becomes a demodulated FM program.
This embodiment of the receiver provides the ability to cancel or filter out a single large undesired signal whose frequency is unknown, changing or both. The tracking delay element notch filter is an adaptable filter which uses a very simple feed back implementation to continuously adjust the center frequency of a notch filter in response to the instantaneous frequency of a predominant input signal. The critical implementation component is a single adjustable delay time.
Cancellation is made possible by delay elements whose relative delays are adjusted quickly and linearly in response to the center frequency and whose amplitude responses may also be adjusted to control the depth of the cancellation. A 180xc2x0 phase detector provides the delay control signal, is relatively easy to implement, may be used in a threshold sense or as a linear detector, and may be adjusted in detection sensitivity to control the loop dynamics which establish the accuracy of frequency tracking.
The tracking notch can track the instantaneous frequency of an FM input signal in real time. The control voltage which causes the frequency of the notch to track the frequency of the predominant incoming waveform tracks the instantaneous frequency of the input signal to control the instantaneous notch frequency. The tracking canceler demodulates FM by estimating instantaneous frequency directly with this voltage rather than indirectly by tracking phase as in a conventional PLL. Phase discontinuities, such as those which may be encountered while moving through a multipath environment, have less effect on a tracking canceler which tracks frequency than on a conventional PLL which tracks phase. The result is an FM demodulator which is more stable than a conventional PLL in a moving multipath environment.
In another aspect of the invention, the IBOC FM-DAB receiver incorporates an FM to AM conversion canceler which functions upon the principle that the amplitude of the FM to AM conversion interference component is correlated to the instantaneous frequency of the FM signal as a function of the channel multipath. The FM to AM conversion canceler estimates this correlation and continuously updates the estimate. A cancellation signal is generated within the receiver which corresponds with the correlation estimate and is used for canceling the effective multipath from the received signal. Three embodiments of the FM to AM conversion canceler are shown. The first provides a base band cancel signal which is generated from a xe2x80x9clook up tablexe2x80x9d. The look up table includes a running estimate of the FM to AM conversion interference to be expected from a given instantaneous FM frequency. A second embodiment based on polynomial channel estimation includes a polynomial generator which is driven by the FM signal frequency for estimation of the FM to AM conversion interference in the channel. This estimate is subtracted from the raw DAB composite as before. The coefficients of the polynomial are derived by measuring and integrating the cross correlation between the resulting DAB composite and each term of the polynomial. In a third embodiment of the FM to AM conversion canceler, polynomial channel estimation is implemented in the intermediate frequency (IF) section of the receiver. For the third embodiment, two separate polynomials are used to continuously estimate and cancel the in-phase and quadrature components of the FM to AM interference caused by multipath.